Combine flour and yeast in a large bowl. Cut in butter using a pastry blender until dough is crumbly. Stir in egg yolks. Blend in sour cream. Flatten dough into disk. Cover with plastic wrap and refrigerate over night. Preheat oven to 350 degrees. Combine walnuts, sugar, and cinnamon in a medium sized bowl. Sprinkle 1/4 of this mixture into an 8 inch circle on a floured board. Divide dough into 4 equal parts. Place 1 part on top of the nut mixture. Roll out dough into a circle, 1/8 inch thick and 9-10 inches in diameter, lifting as you roll and pushing nut mixture back underneath. Cut the circle into 16 equal wedges. Roll up each triangle, starting at the outside edge and ending at the point. Bend the cookie into a crescent shape. Repeat with remaining dough and nut mixture. Place on an ungreased lightweight aluminim cookie sheet. Bake until puffed, golden brown and cooked through, about 18-20 minutes. Cool on wire rack.